The invention is applied for example in intra-operative imaging during a medical intervention. During a medical intervention real-time images (live images) are obtained for navigation of medical instruments e.g. with the aid of fluoroscopic imaging. Compared to 3D angio images, although these fluoroscopy images or 2D images do not show any spatial (3D) details, they are however available more quickly and minimize the exposure to radiation for patient and doctor. Ideally the spatial information is now retrieved by preoperative or intra-operative 3D images, obtained by CT, 3D angio, C-arm CT or MR images being registered with the two-dimensional images. The combination of co-registered 2D and 3D images now allows the doctor better orientation in the volume. This 2D/3D registration comprises two steps.
1. Image Registration:
It must first be determined from which direction a 3D volume must be projected so that it can be made to coincide with the 2D image. There are different approaches to this, which are described for example in “Graeme Patrick Penney, Registration of Tomographic Images to X-ray Projections for Use in Image Guided Interventions, Phd thesis, University College London, CISG, Division of Radiological Sciences, Guy's Hospital, King's College London, London SE1 9RT England, 2000, Pages 36 through 58 and 97 through 160”.
2. Visualization
The second problem is the visualization of the registered images, i.e. the joint presentation of 2D image and projected 3D image. The standard method is the overlay, in which the two images are laid over one another based on different methods, as has already been proposed for example in the older patent application DE 10 2006 003 126.1.
Overlaying two x-ray images enables differences to be shown. Such differences can be transformations, i.e. the two x-ray images are displaced and/or rotated in relation to each other (which can frequently occur when a patient moves), but can also be anatomical changes such as introduced coils or stents for example. The visibility of these differences in the overlay, but also the ability of the differences to be allocated to the initial x-ray images, is greatly dependent on the visualization of the overlay.
One typical application is the 2D/3D fusion applications used in interventional radiology. These combine information from 3D data records with information from live recorded x-ray images (2D). This combination is only valid however provided the 3D data record is registered against the live x-ray image. This registration is invalid as a rule as soon as the patient has moved after the acquisition of the 3D data record. Then a 2D/3D re registration is necessary. To this end an artificial x-ray image (2D) is computed from the 3D data set and this is compared by overlaying with the live x-ray image. The transformation resulting from the patient movement (displacement and/or rotation) between the two images should be perceptible in the overlay. It is then possible to change the orientation of the 3D data record, so that the patient movement is compensated for and differences are no longer to be seen in the overlay. The 3D data record and the live-x-ray image are then correctly registered again.
The overlaying of two x-ray images is currently implemented by means of the Alpha blending method. In this method the two x-ray images IR1 and IR2 are combined with Alpha value α into a new image I:I=(1−α)*IR1+α*IR2, α∈[0,1]  (1)
Variation of the alpha value enables the proportion of x-ray images in the result image to be varied. For α=0.0 only IR1 is visible in the result image, for α=1.0 only IR2 and for α=0.5 the same parts of both x-ray images are contained in the result image. An example for this visualization of the overlay is shown in FIG. 1.